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The geology and geochemistry of the Lumwana Cu (± Co ± U) deposits, NW Zambia

The geology and geochemistry of the Lumwana Cu (± Co ± U) deposits, NW Zambia
The geology and geochemistry of the Lumwana Cu (± Co ± U) deposits, NW Zambia
The Lumwana Cu (± Co ± U) deposits of NW Zambia are large, tabular, disseminated ore bodies, hosted within the Mwombezhi Dome of the Lufilian Arc. The host rocks to the Lumwana deposits are two mineralogically similar but texturally distinct gneisses, a granitic to pegmatitic gneiss and a banded to augen gneiss which both comprise quartz–feldspar ± biotite ± muscovite ± haematite ± amphibole and intervening quartz–feldspar ± biotite schist. The sulphide ore horizons are typically developed within a biotite–muscovite–quartz–kyanite schist, although mineralization locally occurs within internal gneiss units. Contacts between the ore and host rocks are transitional and characterized by a loss of feldspar. Kinematic indicators, such as S-C fabrics and pressure shadows on porphyroblasts, suggest a top to the north shear sense. The sulphides are deformed by a strong shear fabric, enclosed within kyanite or concentrated into low strain zones and pressure shadows around kyanite porphyroblasts. This suggests that the copper mineralization was introduced either syn- or pre-peak metamorphism. In addition to Cu and Co, the ores are also characterized by enrichments in U, V, Ni, Ba and S and small, discrete zones of uranium mineralization, occur adjacent to the hanging wall and footwall of the copper ore bodies or in the immediate footwall to the copper mineralization. Unlike typical Copperbelt mineralization, unmineralized units show very low background copper values. Whole rock geochemical analyses of the interlayered schist and ore schist, compared to the gneiss, show depletions in Ca, Na and Sr and enrichments in Mg and K, consistent with replacement of feldspar by biotite. The mineral chemistry of muscovite, biotite and chlorite reflect changes in the bulk rock chemistry and show consistent increases in X Mg as the schists develop. ?34S for copper sulphides range from +2.3?‰ to +18.5?‰, with pyrite typically restricted to values between +3.9?‰ and +6.2?‰. These values are atypical of sulphides precipitated by bacteriogenic sulphate reduction. ?34S data for Chimiwungo (Cu + Co) show a broader range and increased ?34S values compared to the Malundwe (Cu) mineralization. The Lumwana deposits show many characteristics which distinguish them from classical Copperbelt mineralization and which suggests that they are formed by metasomatic alteration, mineralization and shearing of pre-Katangan basement. Although this style of mineralization is reported elsewhere in the Copperbelt, sometimes associated with the more widely reported stratiform ores of the Lower Roan, none of the previously reported occurrences have so far developed the tonnages of ore reported at Lumwana.
0026-4598
137-153
Bernau, Robin
3967dd33-5898-40c8-b65a-73641003c563
Roberts, Stephen
f095c7ab-a37b-4064-8a41-ae4820832856
Richards, Mike
41d7d6dc-cb9b-4ca6-885c-ccecfb8c7919
Nisbet, Bruce
62c90876-1aec-40f9-9076-33aa25ccb1c9
Boyce, Adrian
a1e40a10-98ba-45a9-8582-6327a14d6072
Nowecki, James
dcc119c3-9e9a-4d0d-bbb9-38e2bbfec97c
Bernau, Robin
3967dd33-5898-40c8-b65a-73641003c563
Roberts, Stephen
f095c7ab-a37b-4064-8a41-ae4820832856
Richards, Mike
41d7d6dc-cb9b-4ca6-885c-ccecfb8c7919
Nisbet, Bruce
62c90876-1aec-40f9-9076-33aa25ccb1c9
Boyce, Adrian
a1e40a10-98ba-45a9-8582-6327a14d6072
Nowecki, James
dcc119c3-9e9a-4d0d-bbb9-38e2bbfec97c

Bernau, Robin, Roberts, Stephen, Richards, Mike, Nisbet, Bruce, Boyce, Adrian and Nowecki, James (2013) The geology and geochemistry of the Lumwana Cu (± Co ± U) deposits, NW Zambia. Mineralium Deposita, 48 (2), 137-153. (doi:10.1007/s00126-012-0424-5).

Record type: Article

Abstract

The Lumwana Cu (± Co ± U) deposits of NW Zambia are large, tabular, disseminated ore bodies, hosted within the Mwombezhi Dome of the Lufilian Arc. The host rocks to the Lumwana deposits are two mineralogically similar but texturally distinct gneisses, a granitic to pegmatitic gneiss and a banded to augen gneiss which both comprise quartz–feldspar ± biotite ± muscovite ± haematite ± amphibole and intervening quartz–feldspar ± biotite schist. The sulphide ore horizons are typically developed within a biotite–muscovite–quartz–kyanite schist, although mineralization locally occurs within internal gneiss units. Contacts between the ore and host rocks are transitional and characterized by a loss of feldspar. Kinematic indicators, such as S-C fabrics and pressure shadows on porphyroblasts, suggest a top to the north shear sense. The sulphides are deformed by a strong shear fabric, enclosed within kyanite or concentrated into low strain zones and pressure shadows around kyanite porphyroblasts. This suggests that the copper mineralization was introduced either syn- or pre-peak metamorphism. In addition to Cu and Co, the ores are also characterized by enrichments in U, V, Ni, Ba and S and small, discrete zones of uranium mineralization, occur adjacent to the hanging wall and footwall of the copper ore bodies or in the immediate footwall to the copper mineralization. Unlike typical Copperbelt mineralization, unmineralized units show very low background copper values. Whole rock geochemical analyses of the interlayered schist and ore schist, compared to the gneiss, show depletions in Ca, Na and Sr and enrichments in Mg and K, consistent with replacement of feldspar by biotite. The mineral chemistry of muscovite, biotite and chlorite reflect changes in the bulk rock chemistry and show consistent increases in X Mg as the schists develop. ?34S for copper sulphides range from +2.3?‰ to +18.5?‰, with pyrite typically restricted to values between +3.9?‰ and +6.2?‰. These values are atypical of sulphides precipitated by bacteriogenic sulphate reduction. ?34S data for Chimiwungo (Cu + Co) show a broader range and increased ?34S values compared to the Malundwe (Cu) mineralization. The Lumwana deposits show many characteristics which distinguish them from classical Copperbelt mineralization and which suggests that they are formed by metasomatic alteration, mineralization and shearing of pre-Katangan basement. Although this style of mineralization is reported elsewhere in the Copperbelt, sometimes associated with the more widely reported stratiform ores of the Lower Roan, none of the previously reported occurrences have so far developed the tonnages of ore reported at Lumwana.

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Published date: February 2013
Organisations: Geochemistry

Identifiers

Local EPrints ID: 349509
URI: https://eprints.soton.ac.uk/id/eprint/349509
ISSN: 0026-4598
PURE UUID: fbe1ba12-a2bf-4e79-b6d4-0efc0d8ce9e6
ORCID for Stephen Roberts: ORCID iD orcid.org/0000-0003-4755-6703

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Date deposited: 06 Mar 2013 13:57
Last modified: 20 Jul 2019 01:26

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